In recent years an expansion of rice (Oryza sativa L.) acreage in Arkansas has resulted in rice being produced on soils with a history of cotton production. Most of these cotton soils have had repeated applications of monosodium methanearsonate (MSMA) as a herbicide. There is some evidence that arsenical residues in the soil can lead to sterility in rice. In an effort to answer this question, we conducted a field experiment on a Crowley silt loam soil (Typic Albaqualf) in 1975 to evaluate the influence of MSMA on plant growth and yield of rice. Treatments included two water management regimes (continuous flood vs. one drain and dry), three levels of MSMA [0, 1.1, and 11.2 kg/ha of arsenic (As)] and 15 rice cultivars and experimental strains differing in susceptibility to straighthead disease (abnormally developed or sterile flowers resulting in reduced grain yields). Soil samples taken 65 days after arsenic application showed that As remained in the rice root zone. Vegetative growth prior to panicle initiation was not affected by any of the treatments; however, visual observations made near maturity showed that where MSMA had induced sterility, the plants were dark green suggestive of straighthead. Reproductive growth as shown by panicle weights and sterility ratings was affected most by high MSMA rates and continuous flooding. Under continuous flood, panicle weights were reduced and sterility ratings increased as MSMA rate increased. This effect was more pronounced in cultivars most susceptible to straighthead and was minimized by draining the flood and drying the soil. This data suggests that the MSMA residues induced straighthead. A significant and similar relationship was found between panicle weights and sterility ratings at maturity for both water management regimes.

Research on response of direct seeded rice (Oryza sativa L.) to interactions of seeding rate and N rate is very limited, although considerable work has been done in the tropical countries with transplanted rice. Understanding responses by U.S. rice cultivars to these factors is important not only for optimizing management, but also because it provides information concerning the need for further improvement of plant type. Therefore, our objective in this study was to determine what factors might limit performance of ‘Starbonnet’ rice when subjected to a range of seeding and N rates. This study was conducted in field plots for 2 years on a Crowley silt loam soil (Typic Albaqualf) at the Univ. of Arkansas Rice Branch Experiment Station, Stuttgart, Ark. A split plot design was used. Time of N application (preplant or topdress) was the main plot and all possible combinations of seeding rates of 67, 135, and 303 kg/ha and N rates of 67, 135, and 202 kg/ha were the subplots. All treatments were replicated four times. The following measurements were taken: grain yield; stand counts prior to tillering and at maturity; blanks per panicle; florets per panicle; dry weight at heading and maturity; percent N in the flag leaf at heading; leaf area index (LAI) at heading Excessive vegetative growth before anthesis limited grain yield from dense plant populations. This was evidenced by high LAI at heading, reduced dry matter accumulation after heading, and adequate N levels in the flag leaf under conditions of dense population. Further indication of the effect of excessive vegetation prior to anthesis was shown by the fact that number of florets per panicle had more influence on grain yield than did blanks per panicle. Increasing the N rate from 135 to 202 kg/ha increased grain yield only for the 67 kg/ha seeding rate in 1972. Time of N application (preplant or topdress) did not influence grain yield. Under the direct seeding conditions of this study initial plant population, rather than tillering, accounted for the number of panicles per unit area. As seeding rate and plant population increased, number of florets and blanks per panicle decreased. Increasing the N rate increased both florets and blanks per panicle. Both increased seeding rate and N rate resulted in increased LAI. Under the conditions of this test the major factors that limited yield under dense population appeared to be light and CO2, since extra N fertilizer was ineffective.

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When rice (Oryza sativa L.) is grown in a direct‐seeded, delayed‐flood system as used in the southern USA, split topdress applications of N fertilizer have proven to be one of the most effective methods for achieving relatively high grain yields and efficient fertilizer‐N uptake. Knowledge concerning the seasonal uptake patterns of fertilizer N applied in split applications is limited. Consequently, a field study was conducted to measure the seasonal uptake of N fertilizer from split applications made at preflood, 1.3‐cm internode elongation (IE), and 14 d after IE (IE + 14 d). To evaluate each fertilizer application, 15N‐labeled urea (2.42 atom% 15N) was applied at preflood (10 g N m−2), IE (5 g N m−2), or IE + 14 d (5 g N m−2). Fertilizer N contained in the soil as NH4, NO3, and organic N were determined periodically after each 15N application, along with the fertilizer and total N in the rice roots, shoots, and panicles. Plant uptake of fertilizer N from the preflood application was essentially complete within 21 d after application (63.2% of the applied N). Nitrogen uptake was essentially complete within 3 d after application of the midseason treatments. Three days after IE, 63.3% of the applied N had been taken up by the plant whereas the plant had taken up 70.1% of the applied N 3 d after the IE + 14 d N application. Approximately 20% of the fertilizer N was recovered in the soil organic fraction at maturity regardless of the time of fertilizer application. Fertilizer N incorporated into the soil organic fraction reached a maximum within 21 d after the preflood treatment.

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